The sensitivity of widely used silver halide photographic materials has increased over the years from an ISO sensitivity of 100 to an ISO sensitivity of greater than 1000. Emulsions containing large silver halide grains, which give greater sensitivity to light, may be used to increase speed; however, such emulsions usually also increase granularity. In addition, certain silver halide emulsions are relatively more difficult to develop depending upon their particular physical or chemical properties. For example, silver halide emulsions with large grains or silver halide grains having a relatively high iodide content, generally develop at slower rates than emulsions having smaller grains or a low iodide content. The preferred emulsions of this invention, i.e. “low-fogging” emulsions are also much slower to develop due to their relative absence of native fog centers and exhibit longer development induction times (the period before noticeable development occurs).
One way of achieving higher speeds in emulsions is to promote chemical development. Different methods to accelerate development of exposed silver halide grains have been realized. For example, U.S. Pat. No. 6,110,657 describes the release of improved types of electron transfer agents (ETA)s of limited diffusibility for development acceleration without an increase in ‘wrong-way’ interimage. These types of compounds are commonly referred to as electron transfer agent releasing couplers or (ETARC)s. As another example, U.S. Pat. No. 5,605,786 describes a method of imagewise release of an ETA where an —O—CO-(T)n-(ETA) group is attached at the coupling-off site of the ETARC. U.S. Pat. Nos. 4,859,578 and 4,912,025 describe silver halide elements comprising a development inhibitor releasing compound and a compound capable of imagewise releasing an electron transfer agent.
It is also known that couplers that release other types of development accelerating fragments can be used to achieve increased photographic speed. In particular, it is known that compounds that release hydrazine derivatives upon reaction with oxidized developer are effective development accelerator releasing couplers or (DARC)s. For example, U.S. Pat. Nos. 4,482,629; 4,820,616; and 4,618,572 all describe the use of DARCs that release hydrazine derivatives, including those in which the hydrazine derivative also bears a silver absorbable moiety.
Fogged grain emulsion technology has had some limited application as a component in multilayer films, primarily in the area of Kodak Ektachrome color reversal films. The role of the fogged grains is to fully develop in the first developer (black and white) and provide a silver ion source for solution physical development (SPD). Increasing the SPD both enhances the shoulder speed and contributes to the sharpening of the reversal toe-both highly desirable effects. Fogged grains can also be used to control interlayer image effects (color correction effects) when they are coated in a receiver layer (one which is influenced by development in another color record) as described in U.S. Pat. Nos. 4,082,553, 4,656,122 describes a reversal element including a blended emulsion layer containing a non-imaging fine-grained emulsion. U.S. Pat. No. 5,399,466 describes the combined effect upon interimage when both fogged grains and development inhibiting releasing couplers are used in reversal films.
In spite of all of the efforts in the industry, however, there is still a continuing need for methods of improving the photographic speed of silver halide emulsions without compromising other performance features of the photographic element.